Add constants and primitive type readonly attributes to Python IDL compiler

This adds constants to the Python IDL compiler.

As there are no files with only constants, to have real test cases I've
also supported primitive type readonly attributes (currently only have
wrapped readonly attributes),
also moving attribute templates into a macro file.

This allows us to move the following 3 files to Python:
* MallocStatistics.idl
* MemoryInfo.idl
* PerformanceNavigation.idl

The Python generates exactly the same code as the Perl.

About 80 IDL files use constants, but usual other elements as well.

BUG=239771

Review URL: https://chromiumcodereview.appspot.com/23068032

git-svn-id: svn://svn.chromium.org/blink/trunk@156763 bbb929c8-8fbe-4397-9dbb-9b2b20218538